It is well known that regulating the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and "shelf-life" of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock inventory. In the food packaging industry, several means for regulating oxygen exposure have already been developed. These means include modified atmosphere packaging (MAP) and oxygen barrier film packaging.
One method currently being used is through "active packaging," whereby the package for the food product is modified in some manner to regulate the food product's exposure to oxygen. See Labuza and Breene, "Application of `Active Packaging` for Improvement of Shelf Life and Nutritional Quality of Fresh and Extended Shelf-Life Foods," Journal of Food Processing and Preservation, Vol. 13, pp. 1-69 (1989). The inclusion of oxygen scavengers within the cavity of the package is one form of active packaging. Typically, such oxygen scavengers are in the form of sachets which contain a composition which scavenges the oxygen through oxidation reactions. One sachet contains iron-based compositions which oxidize to their ferric states. Another type of sachet contains unsaturated fatty acid salts on a particulate adsorbent. See U.S. Pat. No. 4,908,151. Yet another sachet contains metal/polyamide complex. See PCT Application 90/00578.
However, one disadvantage of sachets is the need for additional packaging operations to add the sachet to each package. A further disadvantage arising from the iron-based sachets is that certain atmospheric conditions (e.g., high humidity, low CO.sub.2 level) in the package are sometimes required in order for scavenging to occur at an adequate rate.
Another means for regulating the exposure to oxygen involves incorporating an oxygen scavenger into the packaging structure itself. Through the incorporation of the scavenging material in the package itself rather than by addition of a separate scavenger structure (e.g., a sachet) to the package, a more uniform scavenging effect throughout the package is achieved. This may be especially important where there is restricted air flow inside the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passing through the walls of the package (herein referred to as an "active oxygen barrier"), thereby maintaining the lowest possible oxygen level throughout the package.
One attempt to prepare an oxygen-scavenging wall involves the incorporation of inorganic powders and/or salts. See European Applications 367,835; 366,254; 367,390; and 370,802. However, incorporation of these powders and/or salts causes degradation of the wall's transparency and mechanical properties such as tear strength. In addition, these compounds can lead to processing difficulties, especially in the fabrication of thin layers such as thin films. Even further, the scavenging rates for walls containing these compounds appear to be unsuitable for many commercial oxygen-scavenging applications, e.g. such as those in which sachets are employed.
The oxygen scavenging systems disclosed in European Applications 301,719 and 380,319 as well as disclosed in PCT 90/00578 and 90/00504 illustrate another attempt to produce an oxygen-scavenging wall. These patent applications disclose incorporating a metal catalyst-polyamide oxygen scavenging system into the package wall. However, this system does not exhibit oxygen scavenging at a commercially feasible rate, i.e., a rate suitable for creating an internal oxygen level of less than 0.1% (starting with air) within a period of four weeks or less at room temperature, as is typically required for headspace oxygen scavenging applications. See Mitsubishi Gas Chemical Company, Inc.'s literature titled "AGELESS.RTM. -A New Age in Food Preservation" (date unknown).
Further, in regards to the incorporation of the polyamide/catalyst system into the package wall, polyamides are typically incompatible with the thermoplastic polymers, e.g. ethylene-vinyl acetate copolymers and low density polyethylenes, typically used to make flexible package walls. Even further, when polyamides are used by themselves to make a flexible package wall, they may result in inappropriately stiff structures. Polyamides also incur processing difficulties and higher costs when compared with the costs of thermoplastic polymers typically used to make flexible packaging. Even further, they are sometimes difficult to heat seal. Thus, all of these are factors to consider when selecting materials for packages, especially flexible packages and when selecting systems for reducing oxygen exposure of packaged products.
U.S. Pat. Nos. 3,935,141 (Potts et al.) and 4,983,651 (Griffin), and references cited therein, disclose polymeric compositions which are environmentally degradable materials, i.e., materials designed to undergo total loss of elongation and tensile strength, after the useful lifetime of the package, upon weathering. In Potts (Column 12, line 13), reduction in elongation-at-break to below 20% results in the embrittlement of polyethylene. Such materials (column 1, line 68 through column 2, line 6) "undergo high levels of multifaceted crazing, followed by cracking and ultimately resulting in particulate formation". This strongly implies that along with a loss in elongation, a complete loss of tensile strength occurs. Griffin more extensively describes such loss of tensile strength as (column 6, line 20) "so low that it could not be measured in the customary equipment", and he reiterates that (column 8, line 8) "loss in tensile strength and the reduction of elongation-at-break . . . are commonly observed consequences of . . . oxidative degradation".
On the other hand, loss of tensile strength does not always accompany a loss in elongation-at-break. In such cases the dominant reaction is crosslinking rather than polymer chain scission. See Kagiya, V. T. and Takemoto, K., J. Macromol. Sci.-Chem., A, 1976, 10(5), 795-810, and Goto, K., Plastics (Special Issue), 1991, 42(10), 83-8, 94. In the case of 1,2polybutadiene crosslinking is by far the most dominant reaction, whereas even 1,4-polybutadiene shows significant chain scission. See Rabek, J. F., Luck, J., and Ranby, B, European Polymer Journal, 1979, 1089-1110.
The oxygen scavengers suitable for commercial use in films of the present invention are disclosed in copending U.S. Ser. No. 679,419 filed Apr. 2, 1991, and a method of initiating oxygen scavenging generally is disclosed in U.S. Ser. No. 722,067, filed Jun. 27, 1991. Both applications are incorporated herein by reference as if set forth in full.
Losses in tensile strength and elongation are often found as a result of the oxidation of polymers, and are associated with polymer chain scission and crosslinking processes (see Encycl. Polym. Sci. Eng., Volume 4, pp 637-644). Such losses of elongation and tensile strength are most extensively described in terms of their application to environmentally degradable packaging materials (see U.S. Pat. Nos. 3,935,141 and 4,983,651, and Scott, G., "Delayed Action Photo-Activator for the Degradation of Packaging Polymers", Polymers & Ecological Problems, Plenum Press, 1973, pp 27-43). Such changes in physical properties will lead to losses in impact strength, flex cracking resistance, etc., which are not desirable during the useful lifetime of the package. Use of the additional layers in the package may partly circumvent these effects; however, for the best package performance, materials are needed which do not undergo these losses, while scavenging useful quantities of oxygen. It has been discovered that certain polymers and polymer blends with useful scavenging properties will also retain good physical properties during the lifetime of the package.